This invention relates generally to fiber optic particle motion sensors used for detecting acoustic signals. In particular, this invention relates to a single fiber optic acoustic sensor that combines the low frequency response characteristics of a displacement sensor with the high frequency response characteristics of an accelerometer.
Prior fiber optic particle motion sensor art used for acoustic sensing can be classified in two categories: flexural disk accelerometers and flexural disk displacement sensors, each with significant problems. The flexural disk fiber optic accelerometers suffer from a gain-bandwidth limitation that trades off scale factor sensitivity for wider frequency response, dependent on sensor design. Center supported sensors typically have higher gain but lower resonance frequencies, whereas edge supported sensors have the opposite problem. This could be overcome, somewhat by utilizing a fiber optic displacement sensor (seismic sensor), which operates above its resonance. However, these sensors suffer from a weight penalty. To achieve high gain and acceptable bandwidth, displacement sensors need to be large and highly massive.
This invention is a very broad band fiber optic acoustic sensor system. It combines the low frequency response of a displacement sensor and the high frequency response of an accelerometer in a single sensor system. This approach yields a high gain-bandwidth product sensor system without the need for large, massive sensors.
An acoustic sensor system according to the present invention for measuring parameters of acoustic waves in a selected frequency range comprises a displacement sensor and an acceleration sensor mounted to a common support member. The displacement sensor comprises a first circular flexural disk assembly having a first natural frequency that is below the frequency range of the acoustic waves of interest. The first flexural disk assembly comprises a first set of upper and lower spiral-wound optical fiber coils that are attached to opposite sides of a first flexural disk. The acceleration sensor comprises a second circular flexural disk assembly having a second natural frequency that is greater than the frequency range of the acoustic waves of interest. The second flexural disk assembly comprises a second set of upper and lower spiral-wound optical fiber coils that are attached to opposite sides of a second flexural disk. A fiber optic interferometer system is arranged to provide an optical output signal that is a combination of signals outputted from the displacement sensor and the acceleration sensor.
The displacement sensor preferably further includes an inertia ring mounted to an edge of the first flexural disk to enhance the sensitivity by maintaining the edge of the first flexural disk nearly stationary when an acoustic wave in the selected frequency range is incident upon the housing. The second advantage of the inertia ring is to further reduce the natural frequency of the displacement flexural disk, which effectively extends the low frequency range, and hence, the bandwidth of the sensor.
The support member preferably includes a base and a bolt extending from the base. Central passages are formed in the first and second flexural disks such that they may be mounted on the bolt. A nut and washer are engaged with the bolt to secure the first and second flexural disks to the base with the aid of an intervening cylindrical spacer.
Mass-spring systems of the type represented by both types of flexural disks exhibit common resonant characteristics. At frequencies far below resonance, the mechanical response is in phase with the forced excitation; i.e., in phase with the acoustic signal acting on the case. At frequencies near resonance, the phase response begins to lag the acoustic signal. At resonance, this lag angle is 90xc2x0, and at frequencies much greater than the resonant frequency, the phase response approaches 180xc2x0 with respect to the acoustic signal. The sensor system""s operational band spans frequencies above resonance for the displacement portion of the device (with approximately 180xc2x0 phase shift) and below resonance for the accelerometer portion (near 0xc2x0 phase shift). Therefore, the phase response of the two separate sensing elements are therefore approximately 180xc2x0 apart in the operating region between the two resonant peaks. To allow the individual sensor outputs to be additive for increased scale factor, it is necessary to connect the output of the top coil of the displacement flexural disk to the bottom coil of the accelerometer flexural disk and vice-versa. In a practical sense, this allows the changes in the optical path length of the respective coil windings of both flexural disks to be summed together when both are subjected to a common shortened path length compressive load and conversely, the changes in the optical path length of the opposite coil windings are summed together when subjected to a common elongated path length tensile load. The difference between these two composite path lengths provides the interferometric function that is sensed by the photodetector at the output of the 2xc3x972 coupler.
The interferometer system may alternatively comprise a two-wavelength optical signal source and an optical coupler arranged to receive optical signals outputted from the optical signal source and provide the optical signals into each of the first and second upper optical fiber coils and the first and second lower optical fiber coils. The optical coupler also is arranged to combine optical signals from the first and second upper optical fiber coils and the first and second lower optical fiber coils such that interference between optical signals of a first one of the two wavelengths indicates displacement and interference between optical signals of the other wavelength indicate acceleration. Information from the accelerometer and displacement sensors can be combined electronically, as needed, following demodulation.
Each of the first and second flexural disks preferably includes integrally machined upper and lower rings spaced apart from the support post. The presence of these rings provides a winding hub surface upon which the fiber coil can be directly wound.
The surfaces of the first and second flexural disks preferably include curved grooves extending between the upper or lower rings and the outer edges of the flexural disks. The grooves are arranged to allow lengths of optical fiber adjacent the rings to pass under the optical fiber coils that are formed on the surfaces of the flexural disks, thereby preventing microbend stresses on the fiber while permitting the buried fiber lead to exit the coil tangent to the outer edge of the flexural disk.
An appreciation of the objectives of the present invention and a more complete understanding of its structure and method of operation may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings.